A number of published papers employ an ultraviolet-induced T4 reversion system to explore aspects of UV mutagenesis. Concern about a possible artefact in the measurements, plate reactivation, prompted measurements to determine the extent to which the putative reversion response is independent of the plating density of irradiated particles. All experiments indicate that most of the claimed reversion is a reactivation artefact. In additional tests to recover a mutation in a putative new T4 DNA repair gene, dubbed uvsZ, both the mutation of interest plus a novel T4 strain exhibiting greater than wild-type UV resistance have been recovered. Both mutations will be subjected to genetic and phenotypic analysis.